1. Field
The subject matter disclosed herein relates to determining bitloading to be applied to links to one or more nodes of a multicast group.
2. Information
As the popularity of multimedia has increased, the desire to distribute multimedia content, such as digital video, throughout a home or business using existing coaxial cable has also increased. One means for doing so is enabled by the use of devices developed, for example, according to the Multimedia over Coax Alliance (MoCA). Signals carrying multimedia content may enter a home or building at a point-of-entry (POE). From the POE, and within a home or building, such signals may be distributed to various terminals, or nodes that may be a part of a communication network, such as a broadband cable network (BCN), for example. Such a node may be connected to, or included within, various entities, such as cable converter boxes, televisions, video monitors, cable modems, cable phones, and video game consoles, just to name a few examples.
As defined herein, a multicast refers to the delivery of information to multiple destinations simultaneously. The term “multicast information” refers to the information that is sent to multiple destinations. When multicast information is transmitted, multicast messages carry the multicast information. In one case, multicast information is transmitted to nodes connected to a POE and communicating according to an internet protocol (IP). Nodes that are not identified as recipients of such multicast messages may nevertheless receive such messages. Such nodes may then drop, or otherwise ignore multicast messages not intended for that node.
Communications over a link between two nodes may utilize a signaling technique, such as the technique commonly known as “bitloading”. The term “bitloading” as used herein refers to selecting a particular modulation to be used in communicating over a link between nodes. Such signaling techniques consider the characteristics of a particular link between nodes of the network. Link characteristics include the length, amount of attenuation, amount of noise, and/or any other features of the link between two particular nodes that may affect the transmission of a message between the two nodes. A modulation technique may be chosen, for example, to provide the highest data rate possible for transmitting data over the link, while maintaining an acceptable bit error rate.
Different links typically utilize different modulation schemes because the links may have different physical and/or electrical properties resulting in different characteristics for the link. The characteristics of each link that exists between each pair of nodes in the network will vary because of the paths through, attenuation of, and reflections caused by various elements in the link, such as cables, switches, terminals, connections, and other electrical components in the network. Transmission characteristics of a link may be described in terms of a signal-to-noise ratio (SNR), a bit-error rate (BER), and/or power level of a signal received at a node at one end of the link or the other, for example. It should be noted that the link from a first node to a second node may not have the same characteristics as the link in the reverse direction (from the second node to the first).
In one example of a system for communicating multimedia content within a home or business, communications over a BCN use bitloaded orthogonal frequency division multiplexing (OFDM). In this example, data is modulated on multiple carrier signals with different QAM constellations used on each carrier signal.
A multicast message typically originates at a first node and travels downstream to the other nodes on the network at a rate determined by the capability of a worst-case link between the originating node and each other node in order to ensure that each node will be able to receive the multicast message with the desired bit error rate. However, limiting the multicast message to the bitloading capability of the worst case link (sometimes referred to as the “Greatest Common Denominator” or “GCD”) for each carrier signal means that the likelihood is that many of the carriers will be limited to relatively inefficient modulation (i.e., the ratio of the data to symbols is relatively low or said another way, the amount of data that can be sent using the same bandwidth is relatively low). Accordingly, there is a need for a more efficient way to send multicast information over a network using bitloading.